Here's my experience - the '79-'80 Spirit AMX grill will mount to the Eagle radiator support, but it only uses the center mounting hole on the top part of the grill piece - I had to remove the two outer mounting tabs to get it to fit. There must have been a minor change in the headlight buckets, because my '83 SX/4's buckets do not have provisions for using the outer upper mounting tabs on the AMX grill. Problem with the AMX grill is they're rare and expensive.
I remembed finding this when I changed the headlight bucket in an '86 Eagle. There are different ones and I'm thinking that there are Z-shaped brackets or extensions used on some cars. Does any of this sound familiar? This was all over 20 years ago...